The phenomenon of surface enhanced Raman scattering has been used in the past for various purposes. One example of such use is in connection with a fast intramolecular transistor or switch device, as is disclosed in applicant's commonly assigned copending U.S. patent applications Ser. No. 535,672, filed September 26, 1983 and now abandoned, and Ser. No. 652,317, filed September 19, 1984 and now U.S. Pat. No. 4,855,243, the entire disclosures of which hereby are incorporated by reference. As is disclosed in such applications, light is scattered by a medium onto which incident light is directed; at least one detectable characteristic of the scattered light, such as wavelength or frequency, ordinarily will be a function of a characteristic, such as wavelength or frequency, of the incident radiation and/or of a characteristic of the scattering medium. Using Raman spectrometry techniques, the Raman spectrum of the scattered light can be detected to provide an output function.
Surface enhanced Raman scattering (hereinafter referred to as SERS) phenomena also have been used in the past for analytical purposes, e.g. to detect and to examine characteristics of various materials. SERS effects have been utilized and encountered in the past employing thin layers of one material coated on or supported on a support substrate. Exemplary substrates used in the past include silver, gold and copper. To achieve and/or to increase a detectable SERS effect, it was common practice to roughen the surface of such substrate prior to applying the coating layer thereon. The mechanism causing SERS effect was not well understood in the past, and, therefore, as applicant has discovered, such surface roughening did not necessarily contribute to an effective SERS result.
In prior SERS devices a semiconductor material layer would form on the surface of the substrate; however, the usual procedure for SERS was to remove or to minimize such layer. In the present invention, on the other hand, use is made of such layer as is described further below. Moreover, in the prior SERS devices the surface of the substrate was intentionally roughened, whereas in the present invention there is intentional polishing of such surface both because roughening is unnecessary and because the polished surface may serve as one reflector of an optical cavity, particularly an optically resonant cavity.
It is known in the field of light amplification by stimulated emission of radiation, lasers, that a lasing effect will occur in response to certain inverse population requirements. For example, in a particular material or medium when the population of electrons thereof in a relatively high energy level or energy state exceeds the population of electrons of such material that are a relatively lower energy level or energy state, inverse population requirements for lasing occur. Specifically, as electrons at the higher energy level drop to the relatively lower energy level, such electrons emit photons of light. The frequency of such photons is a function of the energy gap between the relatively high energy level and the relatively lower energy level mentioned.
Such inverse population may be created in a gas by passing a high density current through the gas to induce collisions between molecules or atoms thereof. The collisions result in pushing electrons to higher energy levels or states. When the electrons drop down to the relatively lower energy level, such as the so-called ground state therefor, they emit photons; this is stimulated emission of radiation. As was noted above, the frequency of the photon(s) is a function of the energy gap across which a given electron drops when going from the high energy state to the relatively lower energy state. In liquids such inverse population may be created using collisions induced by high electrical currents and/or by chemical reactions. Inverse population and lasing can be created in a solid by using impurities, such as chromium, in a perfect crystal, such as ruby. The chromium impurities in such example are brought to a higher energy state by applying high density photon flux thereto. The chromium atoms, after absorption of such light (photon flux) assume a relatively higher energy level than the ground state thereof, whereby inverse population conditions are met and subsequent lasing may occur as such chromium atoms drop back to the ground state. Such application of photon flux is known as optical pumping.
In semiconductor materials inverse population conditions can be created by energizing electrons from the valence band to a conduction band and the creation of electron hole pairs. Such energization can be achieved by passing high density currents through the semiconductor material, electron bombardment and/or optical pumping. The lasing effect occurs during the process of electron hole annihilation, i.e. the recombination of an electron and a hole.
In prior semiconductor lasers a semiconductor film was formed on an electrically conductive substrate. Lasing was caused by bombarding the semiconductor material with high energy electrons. However, in such devices the electrically conductive substrate primarily was used for thermal conduction purposes with respect to a cryostat; but the substrate did not contribute to the lasing function. On the other hand, in the present invention the silver substrate is used to contribute to the lasing function by supplying high energy electrons, but bombardment of the semiconductor material to cause lasing is carried out with laser light, not high energy electrons.
Prior semiconductor lasers have required substantial power input to achieve the desired lasing function. Such power requirements not only were expensive to generate and to supply but also resulted in substantial heat generation.
The effect of lasing can be harnessed in a laser by placing the lasing material in an optical cavity defined by reflectors or semi-reflectors at opposite ends of the cavity. The distance between such reflectors typically is a whole number multiple of the wavelength of the emitted light divided by 2. Usually one of the reflectors is a semi-transparent mirror that permits some of the radiation incident thereon to be transmitted therethrough as the laser radiation output of the laser itself.
One aspect of the present invention involves the field of photography. It is known in the field of photography that for each photon impinging on photographic film there are 109 induced changes on the film, a significant amplifying effect.